1. Field of the Invention
The present invention relates to an electroluminescent element and a method for fabricating the same, and more particularly, to an electroluminescent element eliminating the moisture-proof films and a method for fabricating the same.
2. Description of the Prior Art
The conventional electroluminescent element 20 has an electroluminescent laminated portion 27 as shown in FIG. 4. The laminated portion 27 which has a substantially rectangular plane shape is hermetically sealed with covering films 28 and 29 having moisture-proof property such as fluorinated resin.
The electroluminescent laminated portion 27 is formed by laminating a reflective insulating layer 22, a luminescent layer 23 and a transparent electrode 24 on a back electrode 21 successively. The reflective insulating layer 22 is formed by dispersing barium titanate or the like in a binder such as cyanoethyl cellulose or cyanoethyl pullulan. The luminescent layer 23 is formed by dispersing zinc sulfide phosphor particles in a similar binder. That top and bottom of the electroluminescent laminated portion 27 are coated with moisture-proof layers 25 and 26 consisting of moisture-proof films.
However, the electroluminescent element is susceptible under high humidity to the blackening of the phosphor or an increase in the dielectric constant caused by the infiltration of moisture into the interior of the laminated portion 27. For these reasons, it requires moisture-proof films consisting of expensive polychlorotrifluoroethylene or the like and a moisture-absorbing film 6 such as nylon, which results in a very high price of the electroluminescent element itself, and gives rise moreover to such problems as the necessity for securing a margin for sealing, imposition of tight restrictions on the effective luminescent part, difficulty in the reduction of the thickness and the like.
Under these circumstances, there has been proposed an electroluminescent element 30 dispensing with moisture-proof films, having such a structure, as shown in FIG. 5, in which a luminescent layer 32 formed by dispersing in fluorinated resin phosphor particles subjected to moisture-proofing treatment and a reflective insulating layer 33 formed by dispersing the powder of an insulator, such as barium titanate, in a fluorinated resin are formed sequentially by screen printing on a transparent electrode 31. Then a back electrode 34 is printed on the reflective insulating layer 33 and an insulating layer 35 is coated on top of it.
However, since the various layers on the transparent conductive film 31 in such a structure are formed by using screen printing process, there is a flaw in the flatness and the quality of the luminescence is not quite satisfactory. For these reasons, there arises the necessity of forming an undercoating layer consisting of cyanoethyl-pullulan between the transparent electrode and the luminescent layer, as disclosed in Japanese Utility Model Publication No. 5-26720 (1993), and pin-holes tend to be generated in the luminescent layer and the insulating layer which becomes the cause of deterioration in the breakdown voltage, as mentioned in Japanese Unexamined Patent Publication (Kokai) No. 2-276193 (1990). Therefore, there exist such problems as an increase in the manhours for flattening treatment to improve the reliability, nonuniformity in the luminescence due to inhomogeneity or nonuniformity of the printed films, difficulty in making the device large-sized, and the like. In addition, because of the screen printing adopted, it is necessary to repeat printing process to obtain a prescribed thickness, making the mass production difficult. Furthermore, because of the high weight ratio more than six of the phosphor to fluorinated resin, there is a problem in that luminescent layer has poor adhesion to the transparent electrode and tends to breaks away from the electrode.